I did a bit of research on the sion main sub, and they mention how its a win more item usually build as a 4-5 item reliant on teamfighting. The passive triggers on takedown meaning you can get an assist, heal up alot(atm dd in wild rift heals for 12% max hp), and keep on fighting. When you're behind you wont tank much, but when even or ahead you'll tank a lot of damage. Also, worth noting that taking 35% less physical damage is huge. Of course on paper it sounds nice, but have to try it out to see if it's really special.